Transducers and acoustical lenses are commonly used for ultrasonic imaging. Standard transducers comprise a transducing element which is commonly made of a flat piezoelectric material and metallic electrodes on both sides. The lens is located in front of the transducer and is used to focus the mechanical waves produced by the transducer. To operate the transducer, an electrical signal is applied to the electrodes. In response, the transducing element produces mechanical waves. The mechanical waves travel through the lens, strike the object being imaged, and produce echoes. Some of these echoes return through the lens and strike the transducing element. In response to the mechanical waves, the transducing element produces an electrical signal on the electrodes. This electrical signal is used to generate an image.
The lens is used to focus the mechanical waves at a target in the object. To produce high resolution, the waves must be focussed to a narrow, concentrated beam. However, standard transducers produce uniform intensity, plane waves, and these waves, when focussed by a standard converging lens, produce "side lobes" in addition to the desired narrow beam or "main lobe". The intensity pattern is similar to a Bessel function waveform. These sidelobes indicate that mechanical waves are striking regions outside of the target, and thus, generating unwanted echoes. Some of these echoes proceed back to the transducer and are summed with those echoes produced by the main lobe mechanical waves. Standard imaging processors cannot distinguish the unwanted echoes from those of the main lobe. Thus, the echoes from the side lobes distort the image.